Imagers, usually laser imagers, are used to record electronically produced information on photographically sensitive media (e.g. photographic film or paper; or photoconductive drums, as in laser printers). Such devices are used in applications wherein information exists in an electronic form and a visually observable presentation of the information is desired. Typically a single beam of light is directed sequentially across a photosensitive media, in a raster scan fashion. Typically a laser (e.g. a gas laser beam) is used to create the beam.
Such imagers are used to create images when a computer is used to control or create the image information. Examples of the functions which are best done with computer assistance are: (1) size adjustment and cropping; (2) combinations of same or other photos into a layout; (3) addition of text or other graphics to photos; (4) color corrections (e.g. correction for use of daylight film with incandescent lighting); (5) unsharp masking (i.e. electronic sharpening of a photograph); (6) darkness and contrast adjustments; and (7) retouching. Hardcopy images are also needed when printing plates are being prepared, and a proof of the electronic image information must be obtained prior to preparation of printing plates from that same image information. Another example of needs for electronic imagers is in the preparation of medical x-rays (e.g. from CAT [Computerized Axial Tomography] or NMR [Nuclear Magnetic Resonance] scans).
Since laser imagers usually image with a single beam of light, a high relative speed of the beam with respect to the media is generally necessary in order to obtain acceptable overall speeds. One prior design involves wrapping the photosensitive media around the outside of a drum. The drum is spun rapidly while the optical position of the beam is advanced slowly and continuously with a leadscrew.
Another common design involves using a rapidly spinning polygon mirror which deflects the light onto the photosensitive media. The media is usually moved slowly past the optical system in a direction perpendicular to the direction of scan.
The moving element in each system is used to obtain a raster scanning action in one direction. A clock is used to control data flow. If that clock is out of synchronization with the mechanical action, image discontinuities will occur. Due to a number of practical physical limitations in controlling the mechanical actions of these systems, it is often difficult to accurately synchronize the data flow and mechanical action to produce high quality images free from discontinuities.
Specifically, both prior systems require that the rapidly moving component (the drum in the first case, or the polygon mirror in the second case) move with little if any variation in speed. If this is not the case image elements which should line up from scan line to scan line might not do so, but rather appear nonuniform. Industry jargon calls these nonuniformities "jaggies" since lines or object boundaries which should appear smooth appear instead jagged. A second consequence of nonuniform motion would be undesired variations in opacity, especially if the images which are being created are continuous tone images.
It is also required that the slowly moving component (the light beam in the first case, or the media in the second case) move uniformly. If this is not the case, undesired variations in opacity will result. More specifically, increased exposure will result when the scan lines are too close together and decreased exposure will result when the scan lines are too far apart. These conditions would result from media motion which is too slow or too fast, respectively.
In order to minimize these imperfections, various electronic controls and high mechanical precision are typically used in various components in order to synchronize data flow with mechanical action. Considerable cost is involved both in these controls and in the high mechanical precision. Consequently, a need exists in the art for a lower cost synchronization scheme for reducing variations and discontinuities to produce high quality images.